2) There are ________ of breathing
red blood cells in one drop of blood.

a) bunches
b) thousands
c) millions
d) billions

The answer is c) millions.

A single drop of blood contains approximately
4 million red blood cells.
These cells are largely responsible for bringing oxygen to the
tissues
and removing the carbon dioxide waste from the tissues as well.
This process occurs continually as we breathe in and out 24 hours
a day.

It goes on regardless of our state of health.
Even if we have asthma or anemia,
our red blood cells are transferring O2 and CO2
in and out of the body continuously.